Transcript Ultraperipheral Nuclear Collisions

Physics of Ultraperipheral
Nuclear Collisions
Janet Seger
Introduction to UPC physics
 Experimental results from RHIC
 Looking toward the LHC
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May 23, 2008
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Ultraperipheral Nuclear Collisions
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Nuclei miss each other geometrically
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b > R1 + R2
Long-range electromagnetic
interaction
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Exchange of nearly-real photon(s)
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Z
Weizsacker-Williams formalism
Photon flux ~ Z2
b > 2R
Z
Exclusive interaction
Coherent emission limits pT and energy of photon
pT 
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c
 50 MeV/c
RA
k max 
 L c
RA
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Photon interactions
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
Non-pert. QED
 Produces lepton or quark pairs
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Photonuclear
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Vector Meson Dominance
 Photon
fluctuates to a
vector meson (r, w, f)
Vector meson photoproduction -- dominant
coherent process
 Incoherent processes g, q
 Shadowing, exotics
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High Photon Fluxes
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Photon fluxes high at ion
colliders
High probability of multiple
photon exchange
Vector meson can be
accompanied by nuclear
Coulomb excitation
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3- exchange at lowest order
Coulomb excitation  neutrons
Useful for tagging UPCs
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Modeling Photonuclear Interactions
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Photon spectrum:
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Input photon-nucleon data:
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Weizsäcker-Williams
parameterized from results at HERA and fixed target
Scaling p  A:
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Neglecting cross terms -  fluctuates into V which scatters
elastically
Shadowing through a Glauber model
nuclear momentum transfer from form factor
(excellent analytical parameterization)
May 23, 2008
J. Nystrand, S. Klein nucl-ex/9811007
J. Nystrand, S. Klein PRC 60(1999)014903
Klein/Nystrand: Phenomenological model based on scaling
data of p to A
Starlight Monte Carlo agrees well with data
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Starlight predictions
No Breakup
With Breakup (Xn,Xn)
With Breakup (1n,1n)
A.Baltz, S.Klein, J.Nystrand Phys.
Rev. Lett. 89(2002)012301
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Heavy Vector Mesons
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J/Y,U
s(pVp) calculable
from pQCD
2-gluon exchange
Sensitive probe of g(x), g2(x)
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Low-mass states at high rapidity probe low x
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May 23, 2008
Ryskin, Roberts, Martin, Levin, Z. Phys C 76 (1997) 231, Frankfurt LL,
McDermott MF, Strikman M, J. High Energy Physics 02:002 (1999) and
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Martin AD, Ryskin MG, Teubner T Phys.Lett. B454:339 (1999)
Kinematic range of UPCs
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Wp: photon-proton
CM energy
x : Bjorken-x of gluon
Q2 = MV2/4
y=0
J/Y
RHIC
Wp = 25 GeV
x ≈ 2 x 10-2
LHC
PbPb
Wp = 130 GeV
x ≈ 6 x 10-4
U
Wp = 230 GeV
x ≈ 2 x 10-3
U at
LHC
J/Y
at
LHC
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J/Y
at
RHIC
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Blue = impulse
approx.
Red = leading twist
shadowing
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FSZ, Acta Physics Polonica B34
Gluon shadowing suppresses VM
photoproduction
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Gluon shadowing alters rapidity dist.
Black  Impulse
Approx.
Blue  H1 Gluon
density
May 23, 2008
FSZ, Phys Lett B540
Red  Alvero et al.
gluon density
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Experimental Characteristics of UPCs
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Low central multiplicities
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“cleaner” than hadronic collisions
Zero net charge
Low total transverse momentum
Low virtualities
Narrow dN/dy peaked at midrapidity
Large probability of multiple
electromagnetic interactions
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Coulomb excitations
Emission of neutrons
Require: good tracking, particle ID, selective triggering
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Triggering on UPCs
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Typically require
Low multiplicity
 Dissociation of excited nucleus (neutrons in
ZDC)
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 Reduces
statistics but increases triggering
efficiency
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Sometimes include
EM Calorimeter towers for J/psi
 Back-to-back event topology
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UPCs at RHIC
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200 GeV Au-Au collisions
 kmax
~ 3 GeV, WN ~ 35 GeV
 Electron pairs, vector meson photoproduction
studied so far
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Proof of principle for UPC studies
Develop trigger algorithms
 Test UPC models
 Consistent with HERA measurements
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Electron pairs
2-photon interaction
 Za ~ 0.6
 Expect non-perturbative QED effects
Lowest
order
STAR
Higher
order
Pair pT
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Minv
A. J. Baltz, Phys. Rev. Lett. 100,
062302 466 (2008).
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Coherent r photoproduction at
RHIC
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Select coherent events with
pT < 0.15 GeV/c
Mass distribution fit with
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STAR
Breit-Wigner signal
Söding interference term for
direct +- production
Second order polynomial to
describe background
A: amplitude for ρ0
B: amplitude for direct +May 23, 2008
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Many properties consistent with ZEUS
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Ratio of non-resonant to resonant pion production
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200 GeV: |B/A| = 0.84 ± 0.11 GeV -1/2
130 GeV: |B/A| = 0.81 ± 0.28 GeV -1/2
No angular dependence or rapidity dependence
s-channel helicity conservation
Parameter
r0004
e[r1004 ]
r1041
May 23, 2008
STAR
-0.03 ± 0.03 ± 0.06
----
ZEUS
0.01 ± 0.03
0.01 ± 0.02
STAR
-0.01 ± 0.03 ± 0.05
-0.01 ± 0.02
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Incoherent Production
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Extend pT range for measurement
of ρ0 production
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Fit function:
STAR
ds
 a * exp( b * t )  c * exp( d * t )
dt
To the pT2 range: (0.002,0.3) GeV2
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d = 8.8 ±1.0 GeV-2– access to the nucleon form factor
Coherent production
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Coherent
Incoherent production
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Incoherent
b = 388.4 ±24.8 GeV-2 – access to nuclear form factor
s(incoh)/s(coh) ~ 0.29 ±0.03
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Model predictions for r cross
section
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Klein, Nystrand: vector
dominance model (VDM) &
classical mechanical approach for
scattering, based on γp→ρp
experiments results
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Frankfurt, Strikman, Zhalov:
generalized vector dominance
model + Gribov-Glauber
approach
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PRC 60 (1999) 014903
PRC 67 (2003) 034901
Goncalves, Machado: QCD
dipole approach (nuclear effects
and parton saturation
phenomenon)
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Eur.Phys.J. C29 (2003) 271-275
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Energy and A-dependence of r
cross section
62 GeV Au-Au
200 GeV d-Au
STAR
Preliminary
STAR Preliminary
STAR Preliminary
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62 GeV
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Excited r state(s)
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γAu  ρ  π+ π – π+ π –
STAR observes broad peak
around 1510 MeV/c2
May be production of excited
states r(1450) and/or r(1700)
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J/Psi at RHIC (PHENIX)
dN/dmee
(background
subtracted) w/ fit
to (MC) expected
dielectron
continuum and
J/Ψ signals
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D’Enterria, nucl-ex/0601001
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Comparison with Theory
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Large error bars!
Need
more/better data
D’Enterria, nucl-ex/0601001
Strikman, et al., Phys. Lett B626
May 23, 2008
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UPCs at the LHC
2.75 TeV Pb beams
 kmax = 81 GeV, Wp ~ 950 GeV
 Compared to RHIC:
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Greater energy
 Greater photon flux
 Increased cross
sections
 Lower x
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New UPC physics at the LHC
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Elastic Vector Meson production
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+A J/Y +A
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+A U +A
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Photonuclear production of
heavy quarks
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expected prod rate ~ 1x105/ year
sensitive probe of g(x,Q2)
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expected prod rate ~ 1x107/ year
+gcc
Photonuclear jet production;
photon+partonjet+jet; e.g. +g
 q+q
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R. Vogt hep-ph/0407298, M. Strikman, R. Vogt,
S. White PRL 96(2006)082001.
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LHC detectors
Very good tracking, PID
ALICE
Extends to pT =0.05 GeV/c, but |h| < 1
No ZDC trigger
Tracking to |h| < 2.4, but
pT > 0.2 GeV/c
CMS
Good rapidity coverage– can
measure rapidity gaps
ATLAS
Tracking to |h| < 2.4, but
pT > 0.5 GeV/c
Good rapidity coverage– can
measure rapidity gaps
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Conclusions
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UPCs allow study of photon-induced interactions
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RHIC and LHC are high-luminosity A colliders
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RHIC energies comparable to HERA
LHC energies will extend beyond
Experience at RHIC
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Low-multiplicity environment
Can be separated from hadronic background
demonstrated feasibility of UPC studies
Developed trigger algorithms
r and J/Y cross sections
Agreement with HERA results
LHC will probe interesting new physics
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Higher energy, lower x
Shadowing effects, jets
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